An increased understanding of the basic physiology and neurobiology of energy metabolism and homeostasis is critical in the prevention of obesity and eating disorders such as anorexia nervosa. Recent studies indicate that leptin, produced by white adipose tissue, is critical in regulation of energy balance and neuroendocrine function. It is now clear that the central nervous system (CNS), particularly the hypothalamus, is intimately involved in responding to circulating leptin, but the specific neuroanatomy underlying these responses remain poorly characterized. In this proposal, we offer a unique neuroanatomical model of leptin action and describe experiments designed to characterize the role of the dorsomedial hypothalamic nucleus (DMH) in the production of the physiological responses to circulating leptin. We hypothesize that the DMH is a key component of a neuroanatomic pathway which produces leptin responses as the DMH contains leptin receptors, is activated by intravenous leptin, and projects to the paraventricular hypothalamic nucleus (PVH). The PVH is ideally positioned to regulated multiple aspects of responses to leptin and changing energy availability because it possesses chemically and anatomically specific projections to autonomic and endocrine control sites involved in maintenance of homeostasis. This proposal describes anatomic and physiologic experiments designed to critically test specific component of our model. First, we will determine the chemical phenotype of leptin-activated neurons in both fed and fasted rats. This will be accomplished using immunohistochemistry for the FOS protein, and immunohistochemistry or in situ hybridization for neuronal phenotypic markers. Second, using retrograde tracing techniques, immunohistochemistry for the Fos protein, and immunohistochemistry or in situ hybridization for neural phenotypic markers, we will determine the chemical phenotypes of leptin-activated neurons that innervate the PVH. Third, using anterograde tracer injections into the DMH, retrograde tracer injections into the medulla and spinal cord, and immunohistochemistry for Fos and neuronal markers, we will determine whether DMH efferents innervate leptin-activate PVH neurons that project to autonomic preganglionic neurons. Fourth, using a novel experimental preparation which allows us to assess physiological responses and Fos distributions following microinjections into the hypothalamus, we will directly microinject leptin into the DMH, ventromedial hypothalamic nucleus, and arcuate nucleus of the hypothalamus (regions which contain leptin receptors and project to the PVH). These experiments will determine whether activation of leptin receptors in different hypothalamic sites produces distinct physiological responses.